Catalytic Decomposition of Methane to Hydrogen and Carbon

Dec 4, 2008 - Highly active and stable Ni−Cu−SiO2 catalysts are prepared by a .... Ni/SiO2 catalyst could give the carbon yield of 491 gC(gNi)−1...
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Energy & Fuels 2009, 23, 5–13

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Catalytic Decomposition of Methane to Hydrogen and Carbon Nanofibers over Ni-Cu-SiO2 Catalysts Jangam Ashok, Padigapati Shiva Reddy, Gangadhara Raju, Machiraju Subrahmanyam, and Akula Venugopal* Catalysis and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 607, India ReceiVed May 26, 2008. ReVised Manuscript ReceiVed August 18, 2008

Highly active and stable Ni-Cu-SiO2 catalysts are prepared by a coprecipitation method and are employed for direct decomposition of methane to hydrogen and carbon nanofibers at 650 °C and atmospheric pressure. The influence of Cu content is investigated over Ni-Cu-SiO2 samples with different Cu/Si ratios. The activity results revealed that a certain amount of Cu could enhance methane decomposition activity of Ni. The influence of catalyst calcination temperature is also explored, and it is concluded that calcination at 450 °C is enough for good catalytic performance of Ni-Cu-SiO2 samples. The physicochemical characteristics of fresh catalysts are characterized by BET-SA, X-ray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray (SEM-EDX), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) analyses. The deactivated catalysts are analyzed by BET-SA, XRD, Raman, transmission electron microscopy (TEM), and carbon hydrogen nitrogen sulfur (CHNS) techniques. The TEM pictures displayed that the deposited carbon is nanofibers in nature. The Raman spectra distinguished the presence of ordered (G-band) and defective (Dband) carbon and disorders resulting from lattice distortion (D′-band) structures of carbon. TPR analysis revealed the low-temperature reduction of NiO (Ni2+ to Ni0) in the presence of Cu and suggests that Cu produces spillover hydrogen, which considerably accelerates the nucleation of the Ni metal in these reduction conditions and enhances the reducibility of Ni2+. The XPS analysis of Ni-Cu-SiO2 indicated that the main line of Ni 2p at BE of 855 eV implies a change in the chemical state of nickel from NiO to NiSiO3. However, XRD analysis did not show the diffraction lines due to NiSiO3 phase. It is observed that a catalyst composition of Ni-Cu-SiO2 (60:25:15) calcined at 450 °C showed better activity and longevity over the other compositions.

1. Introduction Decomposition of methane into hydrogen and carbon over metal-supported catalysts is of current interest from a viewpoint of an alternative route of hydrogen production from methane, which is a major component of natural gas.1-3 Unlike other challenging processes,4,5 which produce a mixture of hydrogen and carbon oxides, catalytic decomposition of methane (CDM) produces hydrogen and solid carbon thereby eliminating the necessity for the separation of hydrogen from the other gaseous products such as carbon oxides. The COx free hydrogen is highly desirable especially for use in polymer electrolyte membrane (PEM) fuel cell applications. However, the unseparated mixture of hydrogen and methane is a more effective fuel for internal combustion engines and gas turbine power plants than natural or oil gas. From an economical point of view, the feasibility of CDM process is very sensible to the carbon-selling price, which depends on the properties of the carbon obtained. The operation conditions and type of catalysts utilized in the CDM process certainly influence the quality of carbon produced. The utilization of metal-based catalysts leads to the production of carbon * To whom correspondence should be addressed. Telephone: +91-4027193510. Fax: +91-40-27160921. E-mail: [email protected]. (1) Choudhary, T. V.; Aksoylu, E.; Goodman, D. W. Catal. ReV. 2003, 45, 151, and references therein. (2) Muradov, N. Z. Energy Fuels 1998, 12, 41. (3) Zein, S. H. S.; Mohamed, A. R. Energy Fuels 2004, 18, 1336. (4) Lemons, R. A. J. Power Sources 1990, 29, 251. (5) Amphlett, J. C.; Creber, A. M.; Davis, J. M.; Mann, R. F.; Peppley, B. A.; Stokes, D. B. Int. J. Hydrogen Energy 1994, 19, 131.

of high quality whose high selling price would compensate the high cost of the catalyst. The attention of numerous researchers has been paid to carbon materials synthesized by cracking of various hydrocarbons over metal-supported catalysts because of their unique structure built-up by interlaced nanofibers or nanotubes. Properties of these materials are studied regarding their use as adsorbents, catalysts, and catalyst supports or for storage of hydrogen in hydrogen power engineering.6-8 The CDM process over metal-based catalysts is carried out in the absence of O2/air environment and is always accompanied by the catalyst deactivation. It is known that the main reason for deactivation of metal catalysts is carbon formation.9,10 The stability of the catalyst can be improved by choosing a proper support and by addition of promoters to the active component.11 Transition metals such as Ni, Fe, and Co are known to be active for methane decomposition process.12-15 Overall, nickel-based catalysts are more active at low temperatures and provide a higher yield of carbon per mass unit of the active component. (6) Molchanov, V. V.; Chesnokov, V. V.; Buyanov, R. A.; Zaitseva, N. A. Kinet. Katal. 1998, 39, 407. (7) Ashok, J.; Naveen Kumar, S.; Venugopal, A.; Durga kumari, V.; Tripathi, S.; Subrahmanyam, M. Catal. Commun. 2008, 9, 164. (8) Rodriguez, N. M.; Kim, M. S.; Baker, R. T. K. J. Phys. Chem. 1994, 98, 13108. (9) Rostrup-Nielsen, J. R. J. Catal. 1974, 33, 184. (10) Baker, R. T. K. Carbon 1989, 27, 315. (11) Bernardo, C. A.; Alstrup, I.; Rostrup-Nielsen, J. R. J. Catal. 1985, 96, 517. (12) Ashok, J.; Naveen Kumar, S.; Venugopal, A.; Durga Kumari, V.; Subrahmanyam, M. J. Power Sources 2007, 164, 809.

10.1021/ef8003976 CCC: $40.75  2009 American Chemical Society Published on Web 12/04/2008

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Table 1. Physicochemical Characteristics of Fresh Ni-Cu-SiO2 Samples

d

nominal composition (Ni:Cu:Si)

actual compositions (Ni:Cu:Si)a

00:65:35 60:30:10 60:25:15 60:20:20 60:15:25 60:10:30 60:05:35 65:00:35

00:67.3:32.7 61.5:31.6:6.9 60.8:25.7:13.5 59.7:20.6:19.7 62.2:16.0:21.8 60.9:11.6:27.5 58.2:7.5:34.3 65.9:00:34.1

a Obtained from SEM-EDX analysis. Determined from TPR analysis.

b

BET SA (m2/g)

NiO crystallite size (nm)b

Ni domain size (nm)c

H uptakes (mmol/g-cat)d

156 168 203 257 275 281

5.41 4.65 2.89 3.03 2.45 2.12

12.1 8.7 7.4 5.9 6.0 5.2

3.52 2.46 2.01 1.46 1.15 1.12 0.91 1.01

Calculated from XRD spectra of fresh samples.

For this reason, the nickel-metal-based catalysts seem to be the most attractive systems for industrial use. However, nickel-based catalysts are carried out at lower temperatures (550 °C) where the catalysts show lower conversions for a longer time to keep the catalyst active for a longer time.16 These catalysts are very easily deactivated at high temperatures and give lower carbon yield. On the other hand, the methane decomposition process performed at temperatures above 600 °C attracts considerable interest today because the methane conversion is much higher at these temperatures. Hence, the application of alloys as catalysts opens an interesting route to increase the yield of carbon and stability at higher temperatures in methane decomposition.17 According to several publications on Ni-Cu based systems for CDM process, they are more stable and active than Ni based systems.18-22 On the other hand, much work has been done on Al2O3 as a textural promoter for Ni-Cu systems,19 whereas some work has been reported in the utilization of TiO2, MgO, Nb2O5, and SiO2 as textural promoters for CDM process.20-22 Moreover, the influence of different catalyst supports, such as MgO, Al2O3, SiO2, TiO2, and ZrO2, on the Ni activity for methane decomposition is examined, and it is reported that silica is one of the most effective supports studied.23 Takenaka et al. reported that a typical 40 wt % Ni/ SiO2 catalyst could give the carbon yield of 491 gC(gNi)-1 at 500 °C.24 Furthermore, Ermakova et al. reported that the 90 wt % Ni/SiO2 catalyst provided a carbon yield of about 385 gC(gNi)-1 at 550 °C.25 Other supports, such as TiO2, MgO, ZrO2, and Al2O3, gave relatively lower carbon yields.21 Moreover, in alloy-based systems, the 75%Ni-15%Cu-Al2O3 catalyst resulted in high carbon yield of 700 gC(gNi)-1 at 625 °C.18 Furthermore, the 65%Ni-25%Cu-Nb2O5 reached a (13) Venugopal, A.; Naveen Kumar, S.; Ashok, J.; Hari Prasad, D.; Durga Kumari, V.; Prasad, K. B. S.; Subrahmanyam, M. Int. J. Hydrogen Energy 2007, 32, 1782. (14) Ermakova, M. A.; Ermakov, D. Y. Catal. Today 2002, 77, 225. (15) Li, X.; Zhang, Y.; Smith, K. J. Appl. Catal., A: Gen. 2004, 264, 81. (16) Takenaka, S.; Shigeta, Y.; Otsuka, K. Chem. Lett. 2003, 32, 26. (17) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. J. Catal. 2003, 220, 468. (18) Reshetenko, T. V.; Avdeeva, L. B.; Ismagilov, Z. R.; Chuvilin, A. L.; Ushakov, V. A. Appl. Catal., A: Gen. 2003, 247, 51. (19) Echegoyen, Y.; Suelves, I.; Lazaro, M. J.; Moliner, R.; Palacios, J. M. J. Power Sources 2007, 169, 150. (20) Echegoyen, Y.; Suelves, I.; Lazaro, M. J.; Sanjuan, M. L.; Moliner, R. Appl. Catal., A: Gen. 2007, 333, 229. (21) Li, J.; Lu, G.; Li, K.; Wang, W. J. Mol. Catal, A: Chem. 2004, 221, 105. (22) Lazaro, M. J.; Echegoyen, Y.; Suelves, I.; Palacios, J. M.; Moliner, R. Appl. Catal., A: Gen. 2007, 329, 22. (23) Takenaka, S.; Ogihara, H.; Yamanaka, I.; Otsuka, K. Appl. Catal., A: Gen. 2001, 217, 101. (24) Takenaka, S.; Kobayashi, S.; Ogihara, H.; Otsuka, K. J. Catal. 2003, 217, 79. (25) Ermakova, M. A.; Ermakova, D. Y.; Kuvshinov, G. G.; Plyasova, L. M. J. Catal. 1999, 187, 77.

c

Calculated from XRD spectra of reduced samples.

maximum carbon accumulation capacity of 743 gC(gNi)-1 at 600 °C.21 On the contrary, the Ni-Cu-SiO2 (60:25:15) catalyst reported in this work showed a maximum carbon accumulation of 801 gC(gNi)-1 at 650 °C that is prepared by simple coprecipitation method. In our previous work,26 we reported the influence of Cu on methane decomposition performance of Ni-Al2O3 samples; Al2O3 acts as a textural promoter in which hydrotalcite-like structures derived from samples of Ni-Cu-Al2O3 catalysts are prepared by coprecipitation method with changing Cu2+/Al3+ mole compositions keeping Ni content constant. In this paper, we are presenting Ni-Cu systems with SiO2 as a textural promoter. The influence of Cu content and the influence of calcination temperature on methane decomposition activity are tested and correlated with the physicochemical characteristics of the catalysts before as well as after the CDM process at a reaction temperature of 650 °C. 2. Experimental Section The catalysts of Ni-Cu-SiO2 employed in this work were prepared by coprecipitation method. In a typical procedure, the required amount of tetraethyl ortho silicate as a source for silica is placed in a beaker with 500 mL distilled water and is precipitated using the required amount of 0.25 M HCl and is stirred for 1 h. Solution A containing a mixture of metal (Ni and Cu) nitrates and solution B containing a base mixture of 1:1 volume of 2 M NaOH and 1 M Na2CO3 are added slowly and simultaneously to the silica gel precipitate while maintaining a constant pH ∼ 9 throughout the addition. Thus, the produced precipitate is thoroughly washed with distilled water until the pH comes down to the pH of distilled water used for preparation and is dried at 100 °C for 24 h and subsequently is calcined at 400 to 750 °C in static air for 5 h. The samples are represented as Ni-Cu-SiO2 (x:y:z), where x, y, and z are the mole ratios of Ni, Cu, and Si, respectively. The actual metal compositions were measured by scanning electron microscopyenergy dispersive X-ray (SEM-EDX) analysis and are presented in Table 1. X-ray diffraction (XRD) patterns for all the fresh and deactivated samples were obtained on a Rigaku miniflex X-ray diffractometer using Ni filtered Cu KR radiation (λ ) 0.15406 nm) from 2θ ) 2-80°, at a scan rate of 2° min-1, with the beam voltage and a beam current of 30 kV and 15 mA, respectively. The surface areas of the fresh and deactivated catalysts were measured by N2 physical adsorption at -196 °C in an Autosorb-I (Quantachrome) instrument. The specific surface area was calculated by applying the BrunauerEmmett-Teller (BET) method. The scanning electron microscopy (SEM) images of the fresh and deactivated catalysts were recorded using a JEOL-JSM 5600 instrument. For transmission electron microscopy (TEM) analysis, the samples were dispersed in methanol solution and were suspended on a 400 mesh, 3.5 mm diameter Cu grid, and images were taken using JEOL JEM 2010 high-resolution (26) Ashok, J.; Subrahmanyam, M.; Venugopal, A. Int. J. Hydrogen Energy 2008, 33, 2704.

Catalytic Decomposition of Methane transmission electron microscope. X-ray photoelectron spectra (XPS) were recorded using a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer equipped with a Mg anode and a multichannel detector. Charge referencing was done against adventitious carbon (C 1s, 284.8 eV). Shirley-type background was subtracted from the signals. The recorded spectra were always fitted using Gauss-Lorentz curves to determine the binding energies of the different elements. Temperature-programmed reduction (TPR) was carried out in a quartz microreactor interfaced to gas chromatography with thermal conductivity detector (GC with TCD) unit. For TPR analysis, the catalyst sample of about 50 mg was loaded in an isothermal zone of a quartz reactor (i.d. ) 6 mm, length ) 30 cm) heated by an electric furnace at a rate of 10 °C/min to 300 °C in flowing helium gas at a flow rate of 30 cc/min, which facilitates the desorption of physically adsorbed water. Then, after the sample was cooled to room temperature, the helium was switched to 30 cc/min reducing gas of 5% H2 in argon, and the temperature was increased to 700 °C at a rate of 5 °C/min. Hydrogen consumption was measured by analyzing effluent gas by means of thermal conductivity detector. The steam formed during reduction was removed by a molecular sieve trap. Methane decomposition activities over Ni-Cu-SiO2 samples were performed at a reaction temperature of 650 °C and at atmospheric pressure in a fixed-bed vertical quartz reactor (i.d ) 1.0 cm, length ) 46 cm) operated in a down flow mode. Methane supplied by Vadilal Gases Limited (99.99%) was used directly without further purification. Helium was used as a diluent gas. The experimental conditions used were as reported earlier;27 some of them are catalyst weight of 20 mg, methane flow rate ) 30 cc/ min, and helium flow rate ) 30 cc/min, that is, gas hourly space velocity (GHSV) of 180 L (h g-cat)-1. Prior to the reaction, the catalyst was reduced at 550 °C with 5% H2 in N2 for 2 h. The CDM reaction was continued until all the catalysts were deactivated completely. The outflow gas was analyzed by Varian CP-3800 gas chromatograph equipped with a carbosphere column and TCD detector using N2 as a carrier. The concentrations of methane and hydrogen in the product stream were calculated using calibrated data, and then methane conversion (defined as number of moles of methane converted by number of moles of methane fed in) was calculated. The first analysis of the sample was done 5 min after methane was passed over the catalyst, and further analysis was done at regular intervals of 10 min using a six-port auto sampler.

3. Results and Discussion 3.1. CDM Activities over Ni-Cu-SiO2 Catalysts. Methane decomposition activities are performed over Ni-Cu-SiO2 sample at reaction temperature of 650 °C, and all the activity measurements are carried out at similar reaction conditions in order to check the influence of Cu on CDM activity of Ni-Cu-SiO2 catalysts. During the course of the reaction, carbon and hydrogen are regarded as the only products of methane decomposition, and no liquid products are seen. The following reaction occurs during the process. CH4 f C + 2H2 ∆H° ) 75.6 kJ mol-1 In general, Ni and apparently Cu are known to be active components for the decomposition of methane.28 SiO2 is an inactive material for CDM process and is only taken to increase the degree of dispersion of the Ni-containing phases. For the activity profiles of Ni-Cu-SiO2 samples in Figure 1, the amount of Ni is kept constant and the Cu/Si ratios are varied to check the influence of Cu on the extent of dispersion of Ni and its CDM activity. It is observed (Figure 1) that all the catalysts displayed a similar trend of catalytic behavior that is initially (27) Ashok, J.; Naveen Kumar, S.; Subrahmanyam, M.; Venugopal, A. Catal. Lett. 2007, 118, 139. (28) Ammendola, P.; Chirone, R.; Lisi, L.; Ruoppolo, G.; Russo, G. J. Mol. Catal., A: Chem. 2007, 266, 31.

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Figure 1. H2 production rates as a function of time on stream over Ni-Cu-SiO2 catalysts during methane decomposition at 650 °C.

higher conversion rates and rapid decrease in the rate or gradually to some extent with time-on-stream analysis. All the samples sustain CDM reaction for a while but subsequently get deactivated completely. The cause of catalyst deactivation is well documented and is attributed to the deposition of carbon on the catalyst surface/active sites or to accumulation at the entrance of the pores because of pore blockage. Nielsen and Trimm reported that the carbon growth involves a gas-phase reaction on the surface that eventually dissolves in the metal and that would precipitate at a dislocation of the Ni particle to form graphite.29 Furthermore, it has been stated that the carbon filament (CF) formation process is divided into three stages.30 The first step, which is the induction period, involves the dissolution of carbon into the Ni particles leading to the formation of pear-shaped particles in the Ni-alumina catalysts and quasi-octahedral shaped ones in the Ni-Cu-alumina catalysts. The second stage involves the lengthening of the filamentous carbon. Finally, the last stage is catalyst deactivation, which is attributed to the fragmentation or encapsulation of the catalyst particle by carbon. Thus, it is suggested that the deactivation of Ni-Cu-SiO2 samples in CDM process is due to the deposition of carbon leading to the blocking of active component. To compare the overall activity of Ni-Cu-SiO2 catalysts during methane decomposition reaction, parameters such as methane conversions (%), overall carbon accumulation capacity (gC(gNi)-1), and lifetime (min) of the catalysts are considered. Among the tested samples, the Ni-Cu-SiO2 (60:25:15) displayed higher activity with initial conversion of 53%, carbon accumulation of 801 gC(gNi)-1, and a total run time of 1800 min under the reaction conditions maintained in this study. The carbon accumulation capacity of the catalysts with actual Cu content is presented in Figure 2; it shows that the accumulation of carbon increased with an increase in the Cu content up to a mole composition of Ni:Cu:Si of 60:25:15 and with further increase in the Cu content led to a decrease in the carbon yield. The decrease in the catalyst activity is due to a higher amount of copper, which may make the catalyst particles easily become quasi-liquid and which may weaken their stability at high temperatures. Moreover, the presence of Cu in Ni-SiO2 systems significantly enhances its catalytic performance besides the poor catalytic performances of Ni-SiO2 and Cu-SiO2 systems alone (29) Nielsen, J. R.; Trimm, D. L. J. Catal. 1977, 48, 155. (30) Tavares, M. T.; Bernardo, C. A.; Alstrup, I.; Nielsen, J. R. J. Catal. 1986, 100, 545. (31) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1992, 134, 253.

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Ashok et al. Table 2. Physicochemical Characteristics of Fresh Ni-Cu-SiO2 (60:25:15) Sample Calcined at Various Temperatures calcination temp. (°C)

BET SA (m2/g)

NiO crystallite size (nm)a

Ni domain size (nm)b

H uptakes (mmol/g-cat)c

400 450 550 650 750

173 168 151 133 118

4.21 4.65 6.51 9.23 12.15

8.0 8.7 9.2 10.0 11.1

1.55 2.01 1.72 1.48 1.38

a Calculated from XRD spectra of fresh samples. b Calculated from XRD spectra of reduced samples. c Measured from TPR analysis.

Figure 2. Total carbon accumulation capacity of Ni-Cu-SiO2 against actual Cu content measured from SEM-EDX.

Figure 3. Time-on-stream analysis of Ni-Cu-SiO2 (60:25:15) catalysts calcined at different temperatures.

(Figure 1). However, it has been proposed that copper has a high affinity with the graphite structure, which inhibits the formation of a graphite layer on nickel surface. This effect of copper may reduce the growth rate of carbon layers on nickel surface and, therefore, the encapsulation of catalyst particles by carbon layers is retarded, which is regarded as the main reason for the catalyst deactivation. The influence of calcination temperatures over Ni-Cu-SiO2 (60:25:15) catalyst on methane decomposition activity at a reaction temperature of 650 °C is presented in Figure 3 as methane conversions against time on stream. It appeared that the catalyst Ni-Cu-SiO2 (60:25:15) calcined at 450 °C exhibited higher methane decomposition activity with more longevity than other calcined samples. The reason for the low CDM activity for the catalyst calcined at 400 °C is probably because at 400 °C the conversion of the Ni and Cu nitrate salts into NiO and CuO could not be formed.19 Furthermore, the lower activities for the catalysts calcined above 450 °C might be due to the enhancement in the interaction of Ni with SiO2 and the increase in the crystallite size of Ni with the increase in the calcination temperature. The enhancement of interactions between Ni and SiO2 will be discussed in TPR analysis, and the increase in the crystallite size could be confirmed from XRD analysis. 3.2. Catalyst Characterization. The surface areas of fresh Ni-Cu-SiO2 samples are presented in Table 1 along with their nominal and actual mole compositions. It is observed that the surface area of the samples is greatly influenced by the amount of SiO2, that is, the surface area of the samples increased with increase in the SiO2 content, which suggests SiO2 to be the major contributor to the surface areas of the samples. On the other hand, the influence of calcination temperature over surface area of Ni-Cu-SiO2 (60:25:15) sample is presented in Table 2. It seems that the surface area decreased with increase in the calcination temperature, which is due to the sintering of metal oxides during thermal treatment. Figure 4 shows the TPR

Figure 4. The TPR profiles of fresh (a) Ni-SiO2 (65:35), Ni-Cu-SiO2 (60:05:35), (c) Ni-Cu-SiO2 (60:10:30), Ni-Cu-SiO2 (60:15:25), (e) Ni-Cu-SiO2 (60:20:20), Ni-Cu-SiO2 (60:25:15), (g) Ni-Cu-SiO2 (60:30:10), and Cu-SiO2 (65:35) samples calcined at 450 °C.

(b) (d) (f) (h)

patterns of Ni-Cu-SiO2 samples calcined at 450 °C, and their corresponding H2 uptakes are presented in Table 1. The TPR analysis indicates the degree of metallic phase present after activation and heat treatment of the catalysts. The shape of TPR curve assigns the information about the nature of Ni and Cu species. The TPR profiles of Ni-SiO2 and Cu-SiO2 samples are also presented for comparison purpose. The Ni-SiO2 pattern (a) shows a single reduction peak centered at ∼490 °C, which is due to the reduction of bulk NiO species.32 The profiles b-g of Figure 4 show three-stage reduction behavior; initially, a lowtemperature reduction peak has Tmax between 210 and 230 °C, which corresponds to the single-stage reduction of Cu2+ to Cu. Also, a shift in Tmax is observed toward high temperature with an increase in the Cu2+ amount, which could be possibly due to agglomeration of Cu2+ species to form larger crystallites.33 Second, a moderate temperature reduction peak is observed between 260-300 °C that is due to the low-temperature reduction of bulk NiO (Ni2+ f Ni0). The low-temperature reduction Ni2+ is due to the presence of Cu that produces spillover hydrogen, which considerably accelerates the nucleation of the Ni metal in these reduction conditions and which enhances the reducibility of Ni2+.34 Finally, the high-temperature reduction peak observed between 450 and 500 °C corresponds to the reduction of bulk Ni2+ species, which is also observed in Ni-SiO2 (a) profile. The profile (h) of Cu-SiO2 sample showed a single reduction peak centered at ∼270 °C corresponding to reduction of bulk Cu2+ species.35 Figure 5 shows the TPR (32) Rynkowski, J. M.; Paryjczak, T.; Lenik, M. Appl. Catal., A: Gen. 1993, 106, 73. (33) Kim, S. K.; Kim, K. H.; Ihm, S. K. Chemosphere 2007, 68, 287. (34) Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H., Weitkamp, J., Eds.; VCH: Weinheim, Germany, 1997; p 274. (35) Wang, Z.; Liu, Q.; Yu, J.; Wu, T.; Wang, G. Appl. Catal., A: Gen. 2003, 239, 87.

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Figure 5. TPR patterns of fresh Ni-Cu-SiO2 (60:25:15) samples calcined at different temperatures.

profiles of Ni-Cu-SiO2 (60:25:15) catalyst calcined at different temperatures, and its H2 uptakes are presented in Table 2. All the profiles in Figure 5 showed a three-stage reduction behavior similar to Figure 4. However, a shift in the low-temperature reduction peak (because of reduction of Cu2+ f Cu0) toward the high-temperature side with increase in the calcination temperature is observed. The shift is probably due to the increase in the crystallinity of CuO with an increase in calcination temperature. In the case of moderate temperature reduction peak, the shift in Tmax toward the higher side and also an increase in the intensity of the peak are observed with an increase in the calcination temperature. The increase in the intensity is probably due to formation of higher crystallites, and the shift is due to an increase in the interaction of Ni with support material during thermal treatment. The high-temperature reduction peak is also observed at 490 °C, which is due to the reduction of bulk Ni2+ species, which is more prominent in the sample calcined at lower temperature, that is, 400 °C. The powdered XRD patterns for fresh and reduced Ni-Cu-SiO2 samples calcined at 450 °C with varying Cu/Si ratios are presented in Figure 6. The patterns in Figure 6a divulge the presence of NiO as nickel containing phase26 and the reflections as belonging to CuO phase,36 which emerged in the patterns of the samples containing higher Cu/Si mole ratios. The absence of any reflections around 2θ of 22° indicates the presence of SiO2, a textural promoter, in an amorphous phase.37 The crystal domain size of NiO in fresh calcined Ni-Cu-SiO2 samples is determined by broadening of the (111) line of nickel oxide, and the data is reported in Table 1. It is observed that the NiO domain size varied with Cu/Si ratios. On the other hand, Figure 6b presents the XRD patterns of Ni-Cu-SiO2 catalysts after reductive pretreatment at 550 °C for 3 h in H2 stream. In contrast to XRD profiles of fresh catalysts, the presence of metallic Ni, Cu, and Ni-Cu alloy phases may be expected from reduced Ni-Cu-SiO2 catalysts. It shows the presence of reflections at 2θ ) 44.49°, 51.85°, and 76.38°, which correspond to metallic Ni phase [ICDS # 87-0712] only. This metallic Ni comes from the H2 reduction of NiO present in the calcined fresh sample. Furthermore, the intensity of the reflections is increased with increasing Cu/Si ratio similar to the profiles in fresh catalysts. However, the formation of Ni-Cu alloy phase in Ni-Cu-SiO2 catalysts after reductive pretreatment at 550 °C is reported by others,22 but they could not find the Ni-Cu alloy phase in XRD analysis. This supports the results obtained in the present study in which the phase CuO present in the fresh (36) Lazaro, M. J.; Echegoyen, Y.; Suelves, I.; Palacios, J. M.; Moliner, R. Appl. Catal., A: Gen. 2007, 329, 22. (37) Guimon, C.; Auroux, A.; Romero, E.; Monzon, A. Appl. Catal., A: Gen. 2003, 251, 199.

Figure 6. XRD patterns of fresh (a) and after reductive pretreatment of (b) (1) Ni-Cu-SiO2 (60:05:35), (2) Ni-Cu-SiO2 (60:10:30), (3) Ni-Cu-SiO2 (60:15:25), (4) Ni-Cu-SiO2 (60:20:20), (5) Ni-Cu-SiO2 (60:25:15), and (6) Ni-Cu-SiO2 (60:30:10) samples calcined at 450 °C.

Figure 7. XRD patterns of fresh (a) and after reductive pretreatment of (b) Ni-Cu-SiO2 (60:25:15) samples calcined at different temperatures.

samples might be transformed into Ni-Cu alloy during reductive pretreatment. Furthermore, the metallic Ni domain sizes were calculated and are presented in Table 1. Figure 7 shows the powdered XRD patterns of fresh Ni-Cu-SiO2 (60:25:15) samples and after reductive pretreatment of Ni-Cu-SiO2 (60:

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Figure 8. The Ni 2p (a), Cu 2p (b), Si 2p (c), and O 1s (d) XPS profiles of fresh calcined (1) Ni-Cu-SiO2 (60:30:10), (2) Ni-Cu-SiO2 (60: 25:15), (3) Ni-Cu-SiO2 (60:15:25), and (4) Cu SiO2 (65:35) samples.

25:15) samples calcined at different temperatures. Figure 7a reveals the appearance of NiO as the only crystalline nickel containing phase for the catalyst calcined at 400 °C, whereas the patterns of catalysts calcined at temperatures 450 °C and above disclosed the presence of crystalline NiO and CuO phases. However, the crystallinity of metal oxide phase increased consistently with calcination temperature. The calculated NiO crystallite sizes are presented in Table 2. Figure 7b shows the XRD patterns of reduced Ni-Cu-SiO2 (60:25:15) sample

calcined at different temperatures. This metallic Ni phase comes from the reduction of NiO present in the fresh sample during reductive pretreatment. In contrast to fresh XRD patterns, the disappearance of the phase corresponding to Cu might be due to the formation of Ni-Cu alloy. The metallic Ni domain sizes were calculated and are presented in Table 2. Overall, the metallic Ni domain sizes were increased in contrast to their respective NiO crystallite sizes where reductive pretreatment in H2 stream occurred because of thermal sintering.

Catalytic Decomposition of Methane

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Table 3. XPS Binding Energies for Typical Ni-Cu-SiO2 Catalysts Calcined at 450 °C binding energy (eV) catalyst (Ni-Cu-SiO2) Ni 2p3/2 Ni 2p1/2 Cu 2p3/2I Cu 2p1/2 Si 2p 60:30:10

854.7

874.6

934.4

954.2

60:25:15

855.2

872.7

933.6

953.6

60:15:25 00:65:35

855.8

873.6

933.4 934.8

953.3 954.7

O 1s

103.3 531.5 529.3 103.4 531.8 529.8 103.6 532.1 103.9 532.4

The XPS analysis plays an important role in the determination of surface structure, composition, and electronic environment of the catalysts.38 The XPS spectra of Ni 2p, Cu 2p, Si 2p, and O 1s of some of the fresh Ni-Cu-SiO2 samples calcined at 450 °C are displayed in Figure 8. The XPS analysis of Cu-SiO2 sample is also presented in Figure 8 for comparison purpose. Furthermore, the experimental binding energy (BE) values are also given in Table 3. Usually, the Ni 2p and Cu 2p spectra

Table 5. Physicochemical Characteristics of Deactivated Ni-Cu-SiO2 (60:25:15) Sample Calcined at Various Temperatures calcination temp (°C)

BET SA (m2/g)

carbon yield (gC/gNi)a

average width (nm)b

d spacing (nm)b

400 450 550 650 750

220 274 198 154 98

603 801 425 399 207

9.42 8.92 10.29 10.76 11.51

0.337 0.337 0.337 0.336 0.336

a Calculated from CHNS analysis. b Obtained from XRD spectrum of deactivated samples.

Figure 9. XRD patterns of deactivated (1) Ni-Cu-SiO2 (60:05:35), (2) Ni-Cu-SiO2 (60:10:30), (3) Ni-Cu-SiO2 (60:15:25), (4) Ni-Cu-SiO2 (60:20:20), (5) Ni-Cu-SiO2 (60:25:15), and (6) Ni-Cu-SiO2 (60:30:10) samples calcined at 450 °C.

Figure 11. Raman spectra of deactivated (1) Ni-Cu-SiO2 (60:30: 10), (2) Ni-Cu-SiO2 (60:25:15), (3) Ni-Cu-SiO2 (60:20:20), and (4) Ni-Cu-SiO2 (60:15:25) calcined at 450 °C and patterns (5) and (6) of Ni-Cu-SiO2 (60:25:15) catalysts calcined at 400 and 550 °C, respectively.

Figure 10. XRD patterns of deactivated Ni-Cu-SiO2 (60:25:15) samples calcined at different temperatures. Table 4. Physicochemical Characteristics of Deactivated Ni-Cu-SiO2 Samples at Reaction Temperature of 650 °C

b

catalyst Ni-Cu-SiO2 (x:y:z)

BET SA (m2/g)

carbon yield (gC/gNi)a

average width (nm)b

d spacing (nm)b

60:30:10 60:25:15 60:20:20 60:15:25 60:10:30 60:05:35

256 274 236 195 182 121

405 801 544 254 217 116

8.62 8.92 8.97 9.26 9.31 9.10

0.336 0.337 0.337 0.339 0.338 0.339

a Calculated from carbon nitrogen hydrogen sulfur (CHNS) analysis. Obtained from XRD spectrum of deactivated samples.

show a complex structure with intense satellite signals of highbinding energy adjacent to the main peaks, which may be ascribed to multielectron excitation (shake-up peaks).39 The spectra of Ni 2p (in Figure 8a) show a main line at BE of ∼855 eV corresponding to the presence of considerable cationic nickel species on the surface of the catalysts.40 However, the resulting BE of Ni 2p in Figure 8a appeared to be higher than pure NiO (853.8 eV41). This result is in accordance with the conclusion made by Matsumura et al. who indicated that the spectra for Ni 2p could be separated into two Gaussian peaks and that the middle of the broad one is located at 855.9 eV, which nearly equals that of NiSiO3 (856.9 eV).42 This indicated a change in the chemical state of Ni from NiO to NiSiO3, which suggests (38) Alders, D.; Voogt, F. C.; Hibma, T.; Sawatzky, G. A. Phys. ReV. B 1996, 54, 7716. (39) Casella, I. G.; Guascito, M. R.; Sannazzaro, M. G. J. Electroanal. Chem. 1999, 462, 202. (40) Poncelet, G.; Centeno, M. A.; Molina, R. Appl. Catal., A: Gen. 2005, 288, 232.

12

Energy & Fuels, Vol. 23, 2009

Figure 12. TEM images of deactivated (a) Ni-Cu-SiO2 (60:25:15) and (b) Ni-Cu-SiO2 (60:15:25) samples.

that the Ni2+ might have some weak interactions with SiO2. It can be further evidenced from the shift in the binding energy profiles of Ni 2p toward higher BE with an increase in SiO2 content. Figure 8b represents XPS analysis of Cu 2p core level which contains the main peak at BE of ∼933 eV attributed to Cu2+ species.43 However, a shift is observed toward the higher BE side with an increase in the Cu2+ content probably because of the formation of bulk Cu2+ species, and the same is observed from TPR profiles in Figure 4. Although the TPR analysis is a bulk technique, a similar tendency is found with respect to the shift in Tmax because of the formation of Cu2+ species (bulk Cu2+ ions), since Cu2+ gets reduced at high temperatures. The BE of Si 2p XPS spectra in Ni-Cu-SiO2 samples (Figure 8c) is found at ∼103.3 eV ascribed to the presence of Si4+ species.42 However, a shift in the BE is observed toward the higher side with a decrease in the Cu2+/Si4+ ratio probably because of increased interaction of Ni2+ with surface oxygen and Si4+ species. Finally, Figure 8d displays core level O 1s spectra of Ni-Cu-SiO2 samples. It shows a single core level at BE of ∼532 eV for the patterns of Ni-Cu-SiO2 (60:15:25) and Cu-SiO2 samples, whereas two core level features of BE at ∼529.9 and 531.8 eV are observed in the patterns Ni-Cu-SiO2 (60:30:10) and Ni-Cu-SiO2 (60:25:15) samples, respectively. (41) Handbook of X-ray Photoelectron Spectroscopy; Moulder, J. F., Stickle, W. F., Sobol, P. E., Bomben, K. D., Eds.; Perkin-Elmer: Ramsey, MN, 1978. (42) Matsumura, Y.; Tanaka, K.; Tode, N.; Yazawa, T.; Haruta, M. J. Mol. Catal., A: Chem. 2000, 152, 157. (43) Espinos, J. P.; Morales, J.; Barranco, A.; Caballero, A.; Holgado, J. P.; Conzalez-Elipe, A. R. J. Phys. Chem. B 2002, 106, 6921.

Ashok et al.

The appearance of a second low BE at ∼529.9 eV peak is due to the interaction of NiO with CuO (SiO2).44 3.3. Characterization of Deactivated Samples. The structural properties of the deposited carbon have been studied by powder XRD. The XRD patterns of deactivated Ni-Cu-SiO2 samples with varying Cu/Si ratios at a reaction temperature of 650 °C are presented in Figure 9. They reveal the presence of reflection around 2θ of 26.28° attributed to graphitic carbon, and they also show the lines belonging to metallic nickel phase.12 In all the patterns, the reflections of graphitic carbon phase is prominent and dominant over metallic Ni phase. However, they differ in their intensities. The XRD patterns of deactivated Ni-Cu-SiO2 (60:25:15) catalyst calcined at different temperatures are presented in Figure 10, which reveals the presence of reflections corresponding to carbon phase, which is graphitic in nature, and reveals Ni to be in a metallic phase. The phase due to NiO is not observed in either of the XRD patterns of deactivated samples revealing the complete reduction of NiO during the reductive pretreatment with H2 at 550 °C. If any unreduced is left, the generated H2 gas might reduce it during the course of methane decomposition reaction at 650 °C. No peaks corresponding to nickel carbide are detected in any pattern of deactivated samples. Furthermore, the quantitative data can be obtained from further treatment of the powder XRD patterns by Rietveld methods. Table 4 presents the carbon yields, graphene distances (d spacing), and average width of deposited carbon along with the surface areas of deactivated Ni-Cu-SiO2 samples. It shows that the interplanar distances of deposited carbon go from 0.337 nm in sample Ni-Cu-SiO2 (60:30:10) to 0.339 nm in sample Ni-Cu-SiO2 (60:05:35). In general, a perfect graphite has an interplanar distance of basal planes of 0.3354 nm while interplanar distances as high as 0.344 nm have been measured in highly disordered turbostratic carbons in spite of the apparent inherent inaccuracies introduced in dealing with very small domain sizes.45 Additionally, the average width of deposited carbon can vary from 8.82 to 9.1 nm. It is stated that19 the presence of copper in Ni-Al catalysts leads to the formation of a high-ordered carbon structurally close to a perfect graphite while the Ni-Al catalysts alone lead to the formation of a lowordered deposited carbon. This is in good agreement with the results obtained in this work (Table 4) as the sample with a high Cu/Si ratio produces a higher order carbon than the sample having lower Cu/Si composition. Furthermore, the carbon yields, graphene distances, and average width of deposited carbon along with the surface areas of deactivated Ni-Cu-SiO2 (60:25:15) sample calcined at various temperatures are given in Table 5. Lower d spacing values are observed for the samples calcined at higher temperature with high width of deposited carbon corresponding to almost perfect graphite nature of deposited carbon. Raman spectroscopy is one of the most useful techniques for characterizing deposited carbon. The nature of the deposited carbon over Ni-Cu-SiO2 catalysts is characterized by Raman spectroscopy, and the obtained results are presented in Figure 11. The figure displays two distinct bands located at 1320 cm-1 (D-band) and 1580 cm-1 (G-band) attributed either to structural imperfection of graphite or to the presence of nanoparticles and to the in-plane carbon-carbon stretching vibrations rather than the ordered structure carbon, respectively.46 A shoulder peak at around 1600 cm-1 (D′ line) on the G-band is also seen. This peak is induced by disorders resulting from the finite size effect (44) Velu, S.; Suzuki, K.; Vijayaraj, M.; Barman, S.; Gopinath, C. S. Appl. Catal., B: EnViron. 2005, 55, 287. (45) Fujimoto, H. Carbon 2003, 41, 1585.

Catalytic Decomposition of Methane

or lattice distortion.47 It is noted in Figure 11 that the D-band peak is quite more significant than the G-band peak. This indicates that two-dimensional disorders existed in the basal plane, which is quite common in pyrolytic carbon materials synthesized by chemical vapor deposition. The existence of a carbon sheet turbostratic structure in the carbon nanotubes can also lead to a significant D-band peak.48 The morphology of deposited carbon in deactivated samples is studied by TEM analysis. Figure 12 presents TEM images of deactivated (a) Ni-Cu-SiO2 (60:25:15) and (b) Ni-Cu-SiO2 (60:15:25) samples at a reaction temperature of 650 °C. It is evidenced that the decomposition of methane over Ni-Cu-SiO2 catalysts leads to the formation of carbon nanofibers that are a few micrometers in length and that are filamentous in nature. However, the size and length of deposited filamentous carbon varied for various samples. Figure 12 also shows the appearance of bright spots at the tip of carbon filaments with almost the same size of carbon filament because of the presence of Ni particles and is marked with an arrow. This indicates that a Ni metal particle produces a carbon nanofiber of the same size as itself; this corroborates the mechanism which proposed that the Ni metal adsorbs methane and decomposes it into hydrogen and carbon atoms. The CDM process is followed by diffusion of carbon atoms on the metal surface or through the bulk of metal particle to the precipitation sites, where they are crystallized to form a graphitic layer, starting the growth of a carbon nanofibers.10 4. Conclusions

Energy & Fuels, Vol. 23, 2009 13

varied Cu/Si ratios at 650 °C and atmospheric pressure. Decomposition results revealed that the quantity of Cu present in Ni-Cu-SiO2 catalysts significantly influenced methane decomposition activity of Ni. The catalyst Ni-Cu-SiO2 (60: 25:15) calcined at 450 °C/5 h showed high carbon accumulation of 801 gC(gNi)-1 at 650 °C. The TPR analysis indicated that the shift in Tmax is toward low temperature for a moderate temperature peak (i.e., 260-300 °C) as is observed with an increase in the Cu content; it is ascribed to reduction of bulk NiO (Ni2+ f Ni0) which emphasized the role of Cu in enhancing the reducibility of dispersed Ni2+ species. The XRD of fresh samples reveals the presence of crystalline NiO and CuO phases. No SiO2 phase appeared probably because of its amorphous nature. The XRD analysis of deactivated samples revealed that the deposited carbon is graphitic in nature, which is further confirmed by Raman analysis. The Raman spectra distinguished the presence of ordered (G-band) and defective (D-band) carbon and disorders resulting from lattice distortion (D′-band) structures of carbon. The TEM analysis revealed the deposited carbon to be filamentous in nature with nickel particle occupied at the tip of the carbon filament and the particle size being the same as that of carbon nanofiber. Acknowledgment. The authors thank CSIR, New Delhi, for funding this project under NMITLI program. One of the authors, J.A., is grateful to CSIR, New Delhi, for the award of Senior Research Fellowship (SRF).

In summary, methane decomposition activities were carried out over Ni-Cu-SiO2 catalysts having constant Ni amount and

EF8003976

(46) Saito, R.; Kataura, H. Optical Properties and Raman Spectroscopy of Carbon Nanotubes. In Carbon Nanotubes; Dresselhaus, M. S., Dresselhaus, G., Avouris, P., Eds.; Springer-Verlag: Berlin Germany, 2001; p 213.

(47) Liao, K. H.; Ting, J. M. Carbon 2004, 42, 509. (48) Li, W.; Zhang, H.; Wang, C.; Zhang, Y.; Xu, L.; Zhu, K. Appl. Phys. Lett. 1997, 70, 2684.